Bound states of holes in an antiferromagnet

The formation of bound states of holes in an antiferromagnetic spin-1/2 background is studied using numerical techniques applied to the ${\rm t-J}$ Hamiltonian on clusters with up to 26 sites. An analysis of the binding energy as a function of cluster size suggests that a two hole bound state is formed for couplings larger than a ``critical'' value ${\rm J/t]_c}$. The symmetry of the bound state is \dx2y2. We also observed that its ``quasiparticle'' weight ${\rm Z_{2h}}$ (defined in the text), is finite for all values of the coupling ${\rm J/t}$. Thus, in the region ${\rm J/t \geq J/t]_c}$ the bound state of two holes behaves like a quasiparticle with charge $Q=2e$, spin $S=0$, and \dx2y2 internal symmetry. The relation with recent ideas that have suggested the possibility of d-wave pairing in the high temperature cuprate superconductors is briefly discussed.Comment: 12 pages and 3 figures (3 postscript files included), Report LPQTH-93/0

Massive Stars as Major Factories of Galactic Cosmic Rays

The identification of major contributors to the locally observed fluxes of Cosmic Rays (CRs) is a prime objective towards the resolution of the long-standing enigma of CRs. We report on a compelling similarity of the energy and radial distributions of multi-TeV CRs extracted from observations of very high energy (VHE) $\gamma$-rays towards the Galactic Center (GC) and two prominent clusters of young massive stars, Cyg~OB2 and Westerlund~1. This resemblance we interpret as a hint that CRs responsible for the diffuse VHE $\gamma$-ray emission from the GC are accelerated by the ultracompact stellar clusters located in the heart of GC. The derived $1/r$ decrement of the CR density with the distance from a star cluster is a distinct signature of continuous, over a few million years, CR injection into the interstellar medium. The lack of brightening of the $\gamma$-ray images toward the stellar clusters excludes the leptonic origin of $\gamma$-radiation. The hard, $\propto E^{-2.3}$ type power-law energy spectra of parent protons continues up to $\sim$ 1 PeV. The efficiency of conversion of kinetic energy of stellar winds to CRs can be as high as 10 percent implying that the young massive stars may operate as proton PeVatrons with a dominant contribution to the flux of highest energy galactic CRs.Comment: minor revisions have been applied to address the referees' comments, conclusion unchange

Deep inelastic scattering from A=3 nuclei and the neutron structure function

We present a comprehensive analysis of deep inelastic scattering from He-3 and H-3, focusing in particular on the extraction of the free neutron structure function, F_2^n. Nuclear corrections are shown to cancel to within 1-2% for the isospin-weighted ratio of He-3 to H-3 structure functions, which leads to more than an order of magnitude improvement in the current uncertainty on the neutron to proton ratio F_2^n/F_2^p at large x. Theoretical uncertainties originating from the nuclear wave function, including possible non-nucleonic components, are evaluated. Measurement of the He-3 and H-3 structure functions will, in addition, determine the magnitude of the EMC effect in all A < 4 nuclei.Comment: 40 pages, 12 figures, to appear in Phys. Rev.

Clustering aspects in nuclear structure functions

For understanding an anomalous nuclear effect experimentally observed for the beryllium-9 nucleus at the Thomas Jefferson National Accelerator Facility (JLab), clustering aspects are studied in structure functions of deep inelastic lepton-nucleus scattering by using momentum distributions calculated in antisymmetrized (or fermionic) molecular dynamics (AMD) and also in a simple shell model for comparison. According to the AMD, the Be-9 nucleus consists of two alpha-like clusters with a surrounding neutron. The clustering produces high-momentum components in nuclear wave functions, which affects nuclear modifications of the structure functions. We investigated whether clustering features could appear in the structure function F_2 of Be-9 along with studies for other light nuclei. We found that nuclear modifications of F_2 are similar in both AMD and shell models within our simple convolution description although there are slight differences in Be-9. It indicates that the anomalous Be-9 result should be explained by a different mechanism from the nuclear binding and Fermi motion. If nuclear-modification slopes d(F_2^A/F_2^D)/dx are shown by the maximum local densities, the Be-9 anomaly can be explained by the AMD picture, namely by the clustering structure, whereas it certainly cannot be described in the simple shell model. This fact suggests that the large nuclear modification in Be-9 should be explained by large densities in the clusters. For example, internal nucleon structure could be modified in the high-density clusters. The clustering aspect of nuclear structure functions is an unexplored topic which is interesting for future investigations.Comment: 11 pages, LaTeX, 10 eps files, Physical Review C in pres

Tuning Fermi-surface properties through quantum confinement in metallic meta-lattices: New metals from old atoms

We describe a new class of nanoscale structured metals wherein the effects of quantum confinement are combined with dispersive metallic electronic states to induce modifications to the fundamental low-energy microscopic properties of a three-dimensional metal: the density of states, the distribution of Fermi velocities, and the collective electronic response.Comment: 4 pages, 5 figures, to appear in Phys. Rev. Let

Production of (anti-)3^3He and (anti-)3^3H in p-Pb collisions at sNN\sqrt{s_{\rm{NN}}} = 5.02 TeV

The transverse momentum ($p_{\rm{T}}$) differential yields of (anti-)$^3$He and (anti-)$^3$H measured in p-Pb collisions at $\sqrt{s_{\rm{NN}}}$ = 5.02 TeV with ALICE at the Large Hadron Collider (LHC) are presented. The ratios of the $p_{\rm{T}}$-integrated yields of (anti-)$^3$He and (anti-)$^3$H to the proton yields are reported, as well as the $p_{\rm{T}}$ dependence of the coalescence parameters $B_3$ for (anti-)$^3$He and (anti-)$^3$H. For (anti-)$^3$He, the results obtained in four classes of the mean charged-particle multiplicity density are also discussed. These results are compared to predictions from a canonical statistical hadronization model and coalescence approaches. An upper limit on the total yield of $^4\bar{\mathrm{He}}$ is determined.Comment: 30pages, 11 captioned figures, 4 tables, authors from page 25, published version, figures at http://alice-publications.web.cern.ch/node/565